This section provides background information related to the present disclosure which is not necessarily prior art.
A powertrain of this type is known, for example, from U.S. Pat. No. 5,411,110. It provides the operator of the motor vehicle with the option of choosing between a permanent two-wheel drive mode in which the drive of the vehicle takes place only via the primary axle and an automatic four-wheel drive mode, a so-called “on-demand” drive mode, in which under specific driving conditions, for example when the wheels which are driven by the primary axle spin, a specific portion of the drive torque is automatically transferred to the wheels of the secondary axle to provide an intermittent four-wheel drive.
To prevent parts of the powertrain which are not required in permanent two-wheel drive, in particular unnecessary masses, from being moved, a deactuation of the torque transfer section leading to the secondary axle is provided in the powertrain of U.S. Pat. No. 5,411,110 in that the second clutch is disengaged.
As soon as the operator of the motor vehicle selects the automatic four-wheel drive mode, the second clutch is closed. The torque transfer section is now rotationally fixedly connected to the secondary axle so that, on demand, drive torque can be transferred to the secondary axle as fast as possible. In the automatic four-wheel drive mode, the torque transfer section therefore constantly turns along during the travel since it is driven by the drive unit with a closed first clutch and by the secondary axle with an opened first clutch. This is ultimately at the cost of fuel economy.